1. Field of the Invention
The present invention relates to a magnetic storage medium suitable for a magnetic disk unit, and more particularly to a magnetic storage medium excellent in thermal stability of magnetic information stored in the magnetic storage medium.
2. Description of the Prior Art
As recording information, which is dealt with in an information processing apparatus, is increased, it is needed to provide a magnetic storage, which is used as an external storage unit of the information processing apparatus, with a compactness and the more large capacity. For this reason, the magnetic storage needs a magnetic storage medium capable of recording at high recording density. However, according to the conventional CoCr system-alloy of thin film magnetic storage medium, it is known that as magnetic information is recorded at higher recording density, S/Nm of the recorded magnetic information to the regenerative signal is lowered (the medium noise Nm is increased with respect to the output S of the regenerative signal).
A one bit of magnetic information stored in a magnetic storage medium is represented by a direction of a total magnetization consisting of an assembly of the respective magnetizations of a plurality of ferromagnetic crystal particles existing in a one bit cell of the magnetic storage medium. The respective magnetizations of the plurality of ferromagnetic crystal particles are substantially unified in direction in the state that magnetic information is recorded. However, in the event that the magnetization of the adjacent one bit cell is unified in direction opposite to that of the noticed one bit cell, the direction of the magnetization is reversed through a certain width near a boundary between the adjacent one bit cell-to-one bit cell, but not rapidly changed on the boundary. In an area having such a width, magnetizations oriented in mutually different direction are mixed on a zigzag basis. This area is referred to as a magnetization transitional region. One of the causes of occurrence of the medium noise as mentioned above resides in unevenness of a magnetization in the magnetization transitional region. It is known that unevenness of the magnetization occurs owing to a magnetic interaction between ferromagnetic crystal particles indicative of ferromagnetism. In order to weaken the magnetic interaction, there is considered the CoCr system-alloy of thin film magnetic storage medium in which a segregation between a ferromagnetic portion and a non-magnetic portion is promoted in accordance with composition and preparing condition so that the ferromagnetic portion is covered by the non-magnetic portion.
As one which takes the place of the CoCr system-alloy of thin film magnetic storage medium, there is known a granular medium. The granular medium has a structure that the ferromagnetic crystal particles are distributed in the non-magnetic substance such as SiO2. This structure makes it possible to substantially completely divide into parts a magnetic interaction between the ferromagnetic crystal particles. As a result, it is possible to suppress the noise (transitional noise) due to the unevenness of a magnetization in the magnetization transitional region to be substantially zero.
The medium noise occurs also owing to unevenness of a particle size of the ferromagnetic crystal particles. It is considered that the regenerative output is in proportion to the sum total of the volume of ferromagnetic crystal particles. Hence, as the average particle size of one bit cell becomes large, unevenness of a particle size becomes also large. As a result, unevenness of the regenerative output becomes large and thus the medium noise is increased. Therefore, it is considered that the medium noise Nm of the magnetic storage medium is decreased in such a manner that the particle size of the ferromagnetic crystal particles is controlled in the magnetic recording layer of the magnetic storage medium, so that S/Nm is improved.
However, as to the magnetization recorded on ferromagnetic crystal particles in which a magnetic interaction between the ferromagnetic crystal particles is divided into parts so that the ferromagnetic crystal particles are magnetically isolated, as the particle size of the ferromagnetic crystal particles are decreased, energy Kuxc2x7V (anisotropy energyxc3x97volume of particle) representative of a degree of easy orientation of magnetization of the particle in a predetermined direction is reduced. When the energy Kuxc2x7V is reduced, a thermal fluctuation phenomenon wherein a direction of magnetization fluctuates owing to the heat will occur. For that reason, when the particle size is less than a predetermined size, the magnetization of the respective particle involves the thermal fluctuation phenomenon, even at the room temperature. This is associated with a problem that the recording magnetization on one bit cell consisting of the total sum of pieces of magnetization disappears.
In view of the foregoing, it is an object of the present invention to provide a magnetic storage medium capable of recording information at high recording density and also to regenerating the information with a high quality of signal (high S/Nm), and in addition contributing to the elongation of a life span of the recorded information.
To achieve the above-mentioned objects, the present invention provides a magnetic storage medium comprising:
(1) a non-magnetic substrate; and
(2) a recording layer in which grains consisting of a ferromagnetic material are dispersed in an antiferromagnetic material.
In the magnetic storage medium of the present invention as mentioned above, grains consisting of a ferromagnetic material are dispersed in the recording layer of the above item (2). Since the magnetic interaction between the grains is substantially completely divided into part, an unevenness of a magnetization in the magnetization transitional region is small. Thus, according to the magnetic storage medium of the present invention, it is possible to regenerate magnetic information recorded at high recording density with high S/Nm.
In the magnetic storage medium of the present invention as mentioned above, grains consisting of a ferromagnetic material are dispersed in an antiferromagnetic material in the recording layer of the above item (2). Those grains are in contact with the antiferromagnetic material, so that an exchange interaction acts on between a magnetization of those grains and a magnetization of the antiferromagnetic material at an interface of their contact. As a result, magnetic anisotropy energy Ku of those grains is apparently increased. Consequently, the magnetization of those grains is stable to the thermal fluctuation. Thus, magnetic information stored in the magnetic storage medium of the present invention is stable on a thermal basis.
In the magnetic storage medium as mentioned above, it is preferable that said antiferromagnetic material is an oxide, and said ferromagnetic material is a metallic material.
A metal and an oxide are non-solid solution. Thus, according to the magnetic storage medium having the structure as mentioned above, it is possible to expect a favorable separation between grains consisting of the ferromagnetic material in the recording layer of the above item (2).
In the magnetic storage medium as mentioned above, it is preferable that a Nxc3xa9el temperature of said antiferromagnetic material is not less than 400 K.
In the event that a Nxc3xa9el temperature of said antiferromagnetic material is not less than 400 K, generally, even if it is a temperature higher than 60xc2x0 C. or so with which the magnetic storage medium is ensured in use and keeping, the antiferromagnetic material in the recording layer of the above item (2) offers an antiferromagnetism.
In the magnetic storage medium of the present invention as mentioned above, it is preferable that said antiferromagnetic material is NiO.
A Nxc3xa9el temperature of NiO is not less than 400 K, and NiO is an oxide. Accordingly, NiO is suitable for the antiferromagnetic material in the recording layer of the above item (2) of the magnetic storage medium.
In the magnetic storage medium wherein said antiferromagnetic material is an oxide, and said ferromagnetic material is a metallic material, it is preferable that a percentage volume of said antiferromagnetic material in said recording layer is 30 vol. %xcx9c70 vol. %.
In the event that said antiferromagnetic material is provided with the percentage volume as noted above, it is possible to expect a favorable separation between grains consisting of the ferromagnetic material.
In the magnetic storage medium of the present invention as mentioned above, it is preferable that said ferromagnetic material includes Co.
In the event that said ferromagnetic material includes Co, it is possible to obtain a magnetic storage medium which is excellent in an orientation of magnetization and is large in coercive force Hc.
In the magnetic storage medium of the present invention as mentioned above, it is preferable that said ferromagnetic material includes Ni or Fe.
Either of Ni and Fe offers a ferromagnetism and has a large coercive force Hc. This feature is suitable for the magnetic storage medium of the present invention.
In the magnetic storage medium wherein said ferromagnetic material includes Co, it is preferable that a percentage of Pt in said ferromagnetic material is 10 at % xcx9c30 at %, and said ferromagnetic material consists of a CoPt system-alloy.
The anisotropy magnetic field Hk of the ferromagnetic material of the composition referenced above is 4 kOexcx9c8.3 kOe. In the event that the ferromagnetic material has this limit of anisotropy magnetic field Hk, it is possible to obtain an overwrite property of the magnetic storage medium more excellent than that (xe2x88x9225 dB) of a product (FUJITU AL-4) of the applicant""s company.
In the magnetic storage medium wherein said antiferromagnetic material is an oxide, and said ferromagnetic material is a metallic material, it is preferable that said magnetic storage medium further comprises:
(3) a primary layer including a metallic material having a body-centered cubic structure, said primary layer being disposed between said substrate and said recording layer.
According to the magnetic storage medium having the primary layer of the above item (3), the ferromagnetic material of the recording layer of the above item (2) is favorably subjected to a hetero-epitaxial growth on an interface with the primary layer of the above item (3). Thus, it is possible to expect a favorable orientation of a magnetization in the ferromagnetic material.
In the magnetic storage medium further comprising said primary layer including a metallic material having a body-centered cubic structure, it is preferable that said metallic material included in said primary layer is Cr.
In the event that Cr is included in the primary layer of the above item (3), an interval between (110) face-to-(110) face of Cr and an interval between (002) face-to-(002) face of Co, which is used for the recording layer of the above item (2) and is excellent as the ferromagnetic material, are substantially coincident with each other. For this reason, according to the magnetic storage medium as mentioned above, the ferromagnetic material of the recording layer of the above item (2) is subjected to a hetero-epitaxial growth on an interface with the primary layer of the above item (3). Thus, it is possible to expect a favorable orientation of a magnetization in the ferromagnetic material and also to enhance the coercive force Hc of the recording layer.
In the magnetic storage medium wherein said metallic material included in said primary layer is Cr, it is preferable that said primary layer consists of an alloy in which Mo or W is included in Cr, the ferromagnetic material in said recording layer consists of a CoPt system-alloy, and an interval between (110) face-to-(110) face of the alloy constituting said primary layer is larger 1.0%xcx9c2.5% than an interval between (002) face-to-(002) face of the CoPt system-alloy constituting the ferromagnetic material of said recording material.
In the magnetic storage medium as mentioned above, the recording layer of the above item (2) is subjected to a hetero-epitaxial growth on the primary layer of the above item (3) with a scope by 1.0%xcx9c2.5%. Thus, it is possible to enhance the coercive force Hc of the magnetic recording layer 3 and also to provide a favorable orientation of a magnetization in the recording layer.
In the magnetic storage medium as mentioned above it is preferable said magnetic storage medium further comprises:
(4) a protective layer including C, said protective layer is disposed on a top of said recording layer.
In case of this magnetic storage medium, the protective layer of the above item (4) consists of hard particles. Thus, the recording layer of the above item (2) is protected by the protective layer of the above item (4).